This invention relates to circuits for maintaining a single polarity at their output in spite of polarity changes at their input, and in particular to such a circuit which possesses a low impedance.
In telephone systems, subscriber phones are energized by a source of DC potential from the central office. Frequently, the voltage polarity on the lines from the central office is reversed. Thus, some means are required to protect the station set and auxiliary circuitry from such polarity changes. A basic form of such a polarity guard circuit is a standard diode bridge which provides two current paths, one for each polarity, with each path including two forward-biased diodes. The voltage drop across the polarity guard circuit using p-n diodes is therefore typically approximately 1.4 volts. While such voltage drop is acceptable, it is desirable to keep the voltage drop as low as possible to permit a maximum loop length from central office to subscriber set. At the same time, it is also desirable to provide protection of the circuitry from current surges on the line.
In other types of electrical systems, it is desirable to provide an AC signal at the input and get a signal of a single polarity at the output. Such circuits are usually referred to as "full wave rectifiers" but operate in basically the same way as a polarity guard used with a DC signal, except that some sensing of the input polarity is usually provided. In any event, it is also desirable to provide as little voltage drop as possible across the circuits in order to conserve energy.
It is therefore a primary object of the invention to provide a polarity guard or rectifier circuit with a low ON state voltage drop. It is a further object of the invention to provide a polarity guard circuit which also includes surge protection for the devices of the circuit.